DR 704 ensures allocator-aware containers can reuse existing
elements during copy/move assignment, and sequence containers can do
the same for assign().

But apart from std::list (which was changed by DR 320) the sequence
containers define the Effects of assign() in terms of clear() followed
by insert. A user-defined allocator can easily tell whether all old
elements are cleared and then new elements inserted or whether existing elements are assigned
to, so those Effects clauses cannot be ignored via the as-if rule.

The descriptions of the assign() members for deque, forward_list and
vector should be removed. Their intended effects are entirely described by the
sequence container requirements table, and the specific definitions of them are worse than
redundant, they're contradictory (if the operations are defined in terms of erase and
insert then there's no need for elements to be assignable.) The descriptions of
assign() for list are correct but redundant, so should be removed too.